The present invention relates to a color image reading technique, and particularly to a rod lens array as an equi-magnified imaging device constituting an image reading optical system and a unit therefor, and an image scanner (image reading device) using the rod lens array or the unit. Further, the present invention relates to an improvement to achieve high resolution by reducing chromatic aberration.
A rod lens array has been widely used as an equi-magnified imaging device constituting an image reading optical system for an original moving type image scanner (containing an image reader in a facsimile machine, a digital copying machine or the like) because it has advantages that the conjugate length thereof is short, the rod lens array itself is light and it can be manufactured at a low cost.
However, the conventional rod lens arrays have a disadvantage that the chromatic aberration thereof is generally large except for some of them in which the conjugate length is long and the numerical aperture thereof is relatively small (so-called dark). Therefore, these conventional rod lens arrays have been mainly used for monochromatic image reading.
However, color image reading has been recently required, and thus the rod lens arrays have been expected to be used for color image reading in order to positively utilize the above advantages of the rod lens arrays. In order to satisfy this requirement, the chromatic aberration of the rod lens arrays must be reduced.
The focusing parameter g of a square-distribution medium such as a rod lens is given according to the following equation (1):
g2=2[noxe2x88x92n(r)]/(nor2)xe2x80x83xe2x80x83(1)
Here, r represents the distance from the optical axis of the rod lens in the radial direction, no represents the refractive index on the optical axis of the rod lens, and n(r) represents the refractive index at the position which is at the distance r in the radial direction from the optical axis of the rod lens.
The conjugate length TC of an equi-magnified rod lens is given according to the following equation (2):
TC=Zoxe2x88x92[2/(nog)]tan[(Zog)/2]xe2x80x83xe2x80x83(2)
Here, Zo represents the length of the rod lens.
In the equation (1), since both of no and n(r) has wavelength-dependence (wavelength dispersion), g also shows wavelength-dependence. Accordingly, TC calculated according to the equation (2) has wavelength-dependence. Therefore, the imaging position and the magnification are different between light beams which are different in wavelength (color). Accordingly, when color image reading is carried out by using a conventional rod lens array comprising a plurality of rod lenses arranged, the resolution is lowered due to chromatic aberration.
In order to reduce the chromatic aberration, the rod lens has been hitherto manufactured by using materials having low wavelength dispersion. For example, in the case of glass rod lenses formed by an ion exchange method, it has been proposed to reduce the chromatic aberration by suitably selecting the kind and concentration of ions (K. FUJII, MOC [Microoptics Conference]/GRIN""93 KAWASAKI: G13). Further, in the case of plastic rod lenses, the rod lenses are manufactured by using MMA (methyl methacrylate)-based polymer which has relatively small wavelength dispersion of plastics (see Japanese Patent Application Laid-open No. Hei-3-174105).
However, there has not yet been provided any rod lens for which both of reduction in conjugate length and reduction in chromatic aberration (enhancement in resolution) can be performed at the same time and which can be manufactured at low cost.
The resolution can be estimated by measuring MTF (Modulation Transfer Function), for example.
FIG. 31 shows a method of measuring MTF. A rod lens array 101 having a conjugate length TCs at a predetermined wavelength xcexs (the arrangement direction of the rod lenses is vertical to the sheet surface) is put on a measuring apparatus, and a standard rectangular grating 102 and a CCD image sensor array (the arrangement direction of photodetecting elements of the sensor is vertical to the sheet surface) 103 are fixed while the distance TCx between the standard rectangular grating 102 and the CCD image sensor 103 is adjusted to be equal to the conjugate length TCs. The spatial frequency of the standard rectangular grating 102 is set to 6[1 p/mm], for example. Monochromatic light obtained by passing light from a light source (not shown in the figure) through a spectroscope 104 is converted to diffused light by a diffusion plate 106, and then irradiated to the rectangular grating 102. An image of the rectangular grating 102 is focused onto the image sensor array 103 by the rod lens array 101. MTF in each wavelength is measured by varying the wavelength of light emitted from the spectroscope 104.
MTF of each wavelength is obtained by the following equation:
MTF[%]=[(IMAXxe2x88x92IMIN)/(IMAX+IMIN)]xc3x97100
Here, IMAX and IMIN represent the maximum light amount and the minimum light amount measured by the image sensor array 103, respectively.
The definition of MTF and the measuring method as described above are disclosed in Japanese Patent Application Laid-open No. Hei-3-174105.
In general, the refractive index distribution of the rod lens array 101 is not ideal, and thus MTF of an image of the rectangular grating 102 which is focused onto the image sensor array 103 by the rod lens array 101 is not equal to 100%.
FIGS. 32 and 33 show examples of MTF measured by the above method. In both of the examples, the predetermined wavelength xcexs is set to 570 nm, and TCx=TCs is set to 9.1 mm. FIG. 32 shows a measurement result of a glass rod lens array (SLA20D produced by Nippon Sheet Glass Co., Ltd.), and FIG. 33 shows a measurement result of a plastic rod lens array (RA89S produced by Mitsubishi Rayon Co., Ltd.). In all the cases, MTF has the maximum value at the predetermined wavelength xcexs, however, the MTF value is greatly varied depending on the wavelength and the chromatic aberration is large.
Further, not only a white-color light source used in combination with a color image sensor array, but also a three primary color light source including a blue light emission LED, a green light emission LED and a red light emission LED which are used in combination with a monochromatic image sensor array are used as a light source usable in the color image scanner. The light emission spectra of these LEDs are shown in FIG. 34. The light emission peak of the blue light emission LED appears at about 450 nm, the light emission peak of the green light emission LED appears at about 525 nm and the light emission peak of the red light emission LED appears at about 660 nm (the blue light emission LED and the green light emission LED are described in xe2x80x9cApplied Physicsxe2x80x9d, Vol 65. No. 7, 676(1996)). As described above, the wavelength range of the three primary color light source is smaller than the whole range of the visible range, however, there is a 210 nm wavelength interval between the peak wavelength of the blue light emission LED and the red light emission LED. The following table 1 shows MTF values (at 6 [1 p/mm]) of the conventional rod lens arrays having the characteristics shown in FIGS. 32 and 33 at the peak wavelengths of the three primary color light sources.
As is apparent from the table 1, with respect to the conventional rod lens arrays, even when three primary color light sources are used, the MTF values at the wavelength 450 nm are equal to or less than 50% and this indicates that it is insufficient to enhance the resolution of the image scanner.
Therefore, the present invention has been implemented in view of the foregoing situation, and has an object to sufficiently reduce the chromatic aberration of a rod lens array to enhance the resolution when a color image reading operation is carried out by using a rod lens array having a relatively short conjugate length (for example, about 9.1 mm) which is effective in miniaturization and brightness.
In order to attain the above object, according to the present invention, there is provided a rod lens array containing plural kinds of refractive index distribution type rod lenses, characterized in that the plural kinds of refractive index distribution type rod lenses have respective operating wavelength bands and respective predetermined wavelengths which are different between the kinds, each predetermined wavelength being set within each operating wavelength band, and the conjugate length at each predetermined wavelength is set to be substantially equal between the plural kinds of refractive index distribution type rod lenses.
In the rod lens array of an aspect of the present invention, the refractive index distribution type rod lenses are bonded to one another so as to be arranged in parallel to one another and aligned with one another in at least one direction perpendicular to the optical axis direction.
In the rod lens array of an aspect of the present invention, each of the operating wavelength bands is set by effecting passing wavelength band restriction for a color-less rod lens.
In the rod lens array of an aspect of the present invention, each of the operating wavelength bands is set by arranging a passing wavelength band restricting member on the optical path of light passing through a color-less rod lens.
In the rod lens array of an aspect of the present invention, the passing wavelength band restricting member is bonded to the end face of the rod lens.
In the rod lens array of an aspect of the present invention, the passing wavelength band restricting member is disposed away from the end face of the rod lens.
In the rod lens array of an aspect of the present invention, the refractive index distribution type rod lenses are colored.
In the rod lens array of an aspect of the present invention, at least one predetermined wavelength is the peak wavelength at which an emission spectrum of the three primary color light source shows a peak value.
In the rod lens array of an aspect of the present invention, the plural kinds of refractive index distribution type rod lenses are equal to one another in length among all the kinds, and have focusing parameters which are different among the kinds.
In the rod lens array of an aspect of the present invention, the plural kinds of refractive index distribution type rod lenses have the same focusing parameter among all the kinds, and are different in length among the kinds.
In the rod lens array of an aspect of the present invention, the plural kinds of refractive index distribution type rod lenses are arranged so that the same kind of refractive index distribution type rod lenses are aligned with one another in a first direction perpendicular to the optical axis direction, but are not adjacent to one another in a second direction perpendicular to the optical axis direction.
In the rod lens array of an aspect of the present invention, the plural kinds of refractive index distribution type rod lenses are arranged through bulkheads in the second direction.
In the rod lens array of an aspect of the present invention, the plural kinds of refractive index distribution type rod lenses are arranged so that the same kind of refractive index distribution type rod lenses are not adjacent to one another.
In the rod lens array of an aspect of the present invention, the plural kinds of refractive index distribution type rod lenses are arranged in alignment with one another in a first direction perpendicular to the optical axis direction.
Further, in order to attain the above object, according to the present invention, there is provided a rod lens array unit containing plural kinds of refractive index distribution type rod lenses, characterized in that the plural refractive index distribution type rod lenses have respective operating wavelength bands and predetermined wavelengths which are different among the kinds, the conjugate length at each of the predetermined wavelengths different among the kinds is set to be substantially equal among all the kinds, and the plural refractive index distribution type rod lenses are bonded to one another so as to be arranged in parallel to one another.
In the rod lens array unit of an aspect of the present invention, the plural refractive-index distribution type rod lenses are arranged in such a form that the rod lens of the adjacent units can be bonded to one another when plural rod lense array units are repetitively arranged on a plane perpendicular to the optical axis direction.
In the rod lens array unit of an aspect of the present invention, each of the operating wavelength bands is set by effecting passing wavelength band restriction for a color-less rod lens.
In the rod lens array unit of an aspect of the present invention, each of the operating wavelength band is set by bonding a passing wavelength band restricting member to the end face of the color-less rod lens.
In the rod lens array unit of an aspect of the present invention, the refractive index distribution type rod lenses are colored.
In the rod lens array unit of an aspect of the present invention, each predetermined wavelength is set within the corresponding operating wavelength band.
In the rod lens array unit of an aspect of the present invention, the plural kinds of refractive index distribution type rod lenses are equal to one another in length among all the kinds, and have focusing parameters which are different among the kinds.
In the rod lens array unit of an aspect of the present invention, the plural kinds of refractive index distribution type rod lenses have the same focusing parameters among all the kinds, and are different in length among the kinds.
Further, in order to attain the above object, according to the present invention, there is provided an image scanner comprising a light source for irradiating light to an original to be read out, a photodetector for detecting an image of the original which is formed on the basis of light from the original, and the above rod lens array disposed between the original and the photodetector in order to form the image.
In the image scanner of an aspect of the present invention, a three primary color light source is used as the light source.
In the image scanner of an aspect of the present invention, the three primary color light source comprises three primary color LEDs.
In the image scanner of an aspect of the present invention, three kinds of refractive index distribution type rod lenses are used, and each of the predetermined wavelengths of the three kinds of rod lenses is selected within a wavelength area where a power is 1% or more of the respective peak power of an emission spectrum of the three primary color light source.
In the image scanner of an aspect of the present invention, a color image sensor array having three primary color photodetecting elements is used as the photodetector.
In the image scanner of an aspect of the present invention, three kinds of refractive index distribution type rod lenses are used, and each of the predetermined wavelengths of the three kinds of rod lenses are selected within a wavelength area where a sensitivity is 1% or more of the respective peak sensitivity of the three primary color photodetecting elements.
In the image scanner of an aspect of the present invention, two kinds of refractive index distribution type rod lenses are used, and as the predetermined wavelengths of the two kinds of rod lenses are selected from at least one of a first intermediate wavelength and a second intermediate wavelength which are respectively located between respective two adjacent peak wavelengths of three peak wavelengths where the light emission spectrum of the three primary color light source has a peak value. When the first intermediate wavelength or the second intermediate wavelength is adopted as one predetermined wavelength, the peak wavelength other than the adjacent peak wavelengths at both the sides of the intermediate wavelength thus adopted is adopted as the other predetermined wavelength, or a wavelength located away from the peak wavelength concerned with respect to the adopted intermediate wavelength is adopted as the other predetermined wavelength.
That is, in the present invention, by narrowing the operating wavelength band of the rod lens, occurrence of chromatic aberration is reduced. When a wavelength range to be required is broad, the range is divided into plural bands, image focusing is shared by plural kinds of rod lenses having relatively narrow operating wavelengths, and the image focusing characteristic is properly set about each rod lens, thereby substantially reducing the overall chromatic aberration of the array.
Here, the operating wavelength band of a rod lens is defined as a wavelength range in which a light beam emitted from a point on the optical axis of the rod lens passes through the rod lens and arrives at the imaging face to be detected as an image signal at a rate equal to or more than 1% of the maximum value of the image signal, at an equi-magnification focusing state.
The operating wavelength band may be set by disposing a passing wavelength band restricting member comprising a colored member such as a color filter or the like in the optical path at the outside of the rod lens, by coloring the rod lens itself to restrict the passing wavelength band or by using a photosensor having a suitable photosensitivity characteristic.
Any of organic material and inorganic material may be used for the rod lenses.